This proposal focuses on the hypothesis that a lipid signaling pathway on the surface of the mitochondria facilitates the process of mitochondrial fission. We have published evidence that the enzyme Lipin 1, which is recruited to the mitochondrial surface by the lipid phosphatidic acid (PA) via a PA-binding domain in the center of the protein, converts the PA to the related signaling lipid diacylglycerol (DAG), which then promotes mitochondrial fission. Unexpectedly, we also found that Lipin 1 harbors a second, cryptic mitochondrial targeting sequence in the catalytic domain that exhibits highly-specific subcellular localization to sites of future fission events. This led to a model that binding to PA triggers a conformational change to expose the second site which avidly targets fission sites on the mitochondrial tubule and robustly triggers fission in a collaborative but also partially independent manner with Drp1, the dynamic-like protein most widely studied as the physical mediator of fission. This topic has direct clinical significance. Manipulation of mitochondrial fission is being tested for therapeutic application in stroke, cardiac ischemia, and pulmonary hypertension; increased knowledge about mechanisms underlying the fission process will aid in development of these approaches. We propose to pursue areas of interest that have been developed in the context of the Lipin 1 - fission story based on exploration of how DAG triggers fission in collaboration with other components of the fission machinery. Such questions include defining the proteins Lipin 1 interacts with at fission sites or recruits to the fission sites throgh the production of DAG, whether these proteins have roles in the fission process, and whether their function or the fission process itself is driven by DAG production by Lipin 1. We will also examine roles for other members of the Lipin enzyme family in fission, explore how Lipin is recruited to fission sites, and its relationship to the endoplasmic reticulum (ER) and actin cytoskeletal reorganization, which play important roles in the fission process. Taken together, these studies will further our knowledge of the mechanisms underlying fission and be of utility in the development of therapies targeting diseases with connections to this intrinsic process.